Transcript for Ex-Hitman on Bulger: 'You Just Kill Them Before They Kill You'

Now to dramatic testimony in the high-profile trial of reputed boston mob boss white ideal bulger. An ex-hit man on the tan monday to testify against him, a man he was so close to he named his youngest son after bulger. Abc's pierre thomas has the latest and joins us from washington. Good morning, pierre. Reporter: Good morning, robin. As the trial of bulger goes into high gear prosecutors brought in a star witness, a self-avowed hit man who feels betrayed by bulger. James "whitey" bulger is the accused legendary mob boss of boston, a man who's alleged criminal ex-mroirts has been the stuff of movies. Are you still a cop? No. No. No. Reporter: But at his trial monday prosecutors put former hit man john martorano on the stand to prove that bulger's rein was not glamorous but murderous. We had a lot of problems with people. You know, you just kill them before they kill you. Bulger, martorano suggested, did not just order killings, he wanted to be there to witness the bloodshed. Martorano said he gunned down a man named indian al with bulger in a second car watching nearby. Killing was routine. In all martorano murdered at least 20 people. Bulger's defense attorneys portrayed him as a paid liar doing the fbi's bidding because he got a deal from prosecutors. Though the murders were long ago, for surviving family members the pain is till fresh. Sick to my stomach now. Reporter: The jury saw photos of some of the victims, bullet ridden cars and surveillance of bul bulger. He's involved in 19 murders and for much of that he was a paid fbi informa aant ratting out rival mobsters. It sort of broke my heart, he said. They broke all trust that we had, all loyalties and I was just beside myself with it. Prosecutors believe judgment day is finally coming but bulger by all accounts does not look concerned. The most the case has gotten out of him so far, a snicker, robin. Okay, pierre. A snicker, thanks.

This transcript has been automatically generated and may not be 100% accurate.

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